Without a value for x, 4x + x is equal to 5x. If a variable is by itself, you can put a 1 as a coefficient. For example, g = 1g.
1g simply means 1 times g, which is the same as just g.
You cannot add 4x and 1 since they are not like terms. If you have a value for x, THEN you can add them, after substituting the value of x into x.